Why does the ATM not accept my transaction transaction?

Some ideas:

1. You have run out of money in your account.

2. Your card is broken.

3. The ATM is broken.

4. Your bank's IT systems are broken.

5. Other.

Other could be unusual purchases on your card, meaning someone else is using it.

call your bank and ask why your card isn't working...

What message is given on your receipt?

Sounds like the machine is down for everyone, not just you.

I was refused once because I hadn't noticed the card was expired.

Citibank is currently undergoing some extreme problems right now. If you are using one of their cards, nothing would surprise me right now.

On the bottom of the screen where it said that my transaction was not accepted, it showed an error code. I don't remember what the code number was, just that I remember reading the word "ERROR". Maybe it might be an error on the ATM itself?

Most likely, yes, or your bank's system. Did you try a different ATM?

Have you spoken to your bank? That is the first thing you must do, your account may have been debited and you didn't receive the money. Also they'll be able to tell you if there's a problem with your account.

My card was withheld after I requested £50, all I got was a printed receipt that said, "Thank you." It was only when I phoned my bank I found out that the £50 had been debited and then immediately refunded and that there was nothing wrong with my account, it was the machine.

Only your bank can answer the specific questions you need to ask, all people on A2K can do is speculate.